The human immunodeficiency virus type 1 (HIV-1) has emerged in the form of many different strains termed subtypes (A-K) throughout the globe. While all subtypes are pathogenic, subtypes C and A have demonstrated peculiar evolutionary patterns, in terms of their global and in vivo distribution. As these subtypes comprise approximately 85-90% of all HIV infections world-wide, understanding their biology and pathogenic potential may aid in our efforts to develop effective antiviral agents including vaccines. To address this issue, this study will examine primary HIV-1 subtype C and A isolates in vitro according to cytopathicity, viral entry characteristics and resistance to antiviral properties of various entry and fusion based inhibitors